1. Technical Field
The present invention relates generally to cellular wireless communication systems, and more particularly to a system and method to perform DC compensation on a radio frequency (RF) burst in a cellular wireless network.
2. Related Art
Cellular wireless communication systems support wireless communication services in many populated areas of the world. While cellular wireless communication systems were initially constructed to service voice communications, they are now called upon to support data communications as well. The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, cellular wireless users now demand that their wireless units also support data communications. Many wireless subscribers now expect to be able to “surf” the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. The demand for wireless communication system data communications will only increase with time. Thus, cellular wireless communication systems are currently being created/modified to service these burgeoning data communication demands.
Cellular wireless networks include a “network infrastructure” that wirelessly communicates with wireless terminals within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC). Each BSC also typically directly or indirectly couples to the Internet.
In operation, each base station communicates with a plurality of wireless terminals operating in its cell/sectors. A BSC coupled to the base station routes voice communications between the MSC and a serving base station. The MSC routes voice communications to another MSC or to the PSTN. Typically, BSCs route data communications between a servicing base station and a packet data network that may include or couple to the Internet. Transmissions from base stations to wireless terminals are referred to as “forward link” transmissions while transmissions from wireless terminals to base stations are referred to as “reverse link” transmissions. The volume of data transmitted on the forward link typically exceeds the volume of data transmitted on the reverse link. Such is the case because data users typically issue commands to request data from data sources, e.g., web servers, and the web servers provide the data to the wireless terminals. The great number of wireless terminals communicating with a single base station forces the need to divide the forward and reverse link transmission times amongst the various wireless terminals.
Wireless links between base stations and their serviced wireless terminals typically operate according to one (or more) of a plurality of operating standards. These operating standards define the manner in which the wireless link may be allocated, setup, serviced and torn down. One popular cellular standard is the Global System for Mobile telecommunications (GSM) standard. The GSM standard, or simply GSM, is predominant in Europe and is in use around the globe. While GSM originally serviced only voice communications, it has been modified to also service data communications. GSM General Packet Radio Service (GPRS) operations and the Enhanced Data rates for GSM (or Global) Evolution (EDGE) operations coexist with GSM by sharing the channel bandwidth, slot structure, and slot timing of the GSM standard. GPRS operations and EDGE operations may also serve as migration paths for other standards as well, e.g., IS-136 and Pacific Digital Cellular (PDC).
The GSM standard specifies communications in a time divided format (in multiple channels). The GSM standard specifies a 20 ms frame that is divided into four sub-frames, each including eight slots of approximately 625 μs in duration. Each slot corresponds to a Radio Frequency (RF) burst having a left side, a midamble, and a right side. The midamble typically contains a training sequence whose exact configuration depends on modulation format used. Each set of four bursts on the forward link carry a partial link layer data block, a full link layer data block, or multiple link layer data blocks. Also included in these four bursts is control information intended for not only the wireless terminal for which the data block is intended but for other wireless terminals as well.
GPRS and EDGE include multiple coding/puncturing schemes and multiple modulation formats, e.g., Gaussian Minimum Shift Keying (GMSK) modulation or Eight Phase Shift Keying (8PSK) modulation. Particular coding/puncturing schemes and modulation formats used at any time depend upon the quality of a servicing forward link channel, e.g., Signal-to-Noise-Ratio (SNR) or Signal-to-Interference-Ratio (SIR) of the channel, Bit Error Rate of the channel, Block Error Rate of the channel, etc. As multiple modulation formats may be used for any RF burst, wireless terminals must be able to identify the modulation format of any RF burst for successful demodulation and receipt of the RF burst. Further, the modulation format used may cause the processing of the RF burst to vary and requires different signal and channel conditions. For example, the SNR/SIR requirements of the GMSK and 8PSK formats may vary. The 8PSK format needs a higher SNR being associated with the processed RF burst. More robust modulation schemes, such as GMSK, are typically used for noisy channels. Less robust modulation schemes, such as 8PSK, are typically used in less noisy channels. As 8PSK is more sensitive to DC offset and DC offset may vary significantly from one RF burst to another, simple averaging of the received signal over multiple bursts does not adequately estimate the DC offset when using the 8PSK modulation scheme. Therefore, a need exists for improved DC offset compensation for 8PSK. Additionally, different processes may be desired to process signals having different modulation formats.
Furthermore, proper DC offset compensation is crucial for direct conversion (or homodyne) receiver (DCR) architecture. The main feature of DCR is the down-conversion of the radio signal to baseband without any use of intermediate frequencies (IF). The removal of IF would reduce component counts as the need for an IF SAW filter or a second local oscillator for the second frequency translation is eliminated within a smaller form factor. Unfortunately, DCR architectures suffer from DC offset that is a by-product of the direction conversion process. Three main sources for DC offset exist in RF circuits as follows: (1) Local oscillator (LO) signal leaking to, and reflecting off, the antenna and self-down converting to DC through the mixer, (2) a larger near-channel interferer which is leaking into the LO and which also down-converts to DC, and (3) transistor mismatch in the signal path. The leakage due to (2) and (3) can be reduced to some extent by careful front-end design. Nevertheless, if the DC offset is not completely eliminated in the receiver front-end, then the remaining DC offset has to be taken care of in the baseband processing.
While the training sequence assists in processing the RF burst, properly selecting and processing the RF burst according to the modulation format, particularly in an environment where the modulation format may vary is problematic. The wireless terminal needs to immediately identify the modulation format in order to properly process the RF burst and effect proper communications. Thus, a need exists for a means to quickly and efficiently identify the modulation format for the wireless terminal without the wireless terminal performing unnecessary data processing. Once the modulation format is identified, the proper methodology to process the RF burst, including how to perform DC offset compensation, must be quickly effected. When the modulation format is identified as being 8PSK, a need exists for improved estimation of the DC offset as 8PSK is more sensitive to DC offset and the DC offset within 8PSK varies significantly from one RF burst to another. Therefore, a need exists for improved adaptive DC offset compensation methodology for use with the 8PSK modulation scheme.